Textbook of normal histology: including an account of the development of the tissues and of the organs . odermic tracts, since, withthe exception of the epithe-lium of the greater part ofthe genito-urinary organs,the epithelia are the directdescendants of the outer andinner embryonic layers. Thecells lining the passages con-nected with the sexual glands,as well as the urinary tract as far as the bladder, are derived fromthose of the Wolffian body and duct, and hence have, with theselatter, a common mesoblastic origin. The simple arrangement ofthe cells in the earlier stages gradually gives pla
Textbook of normal histology: including an account of the development of the tissues and of the organs . odermic tracts, since, withthe exception of the epithe-lium of the greater part ofthe genito-urinary organs,the epithelia are the directdescendants of the outer andinner embryonic layers. Thecells lining the passages con-nected with the sexual glands,as well as the urinary tract as far as the bladder, are derived fromthose of the Wolffian body and duct, and hence have, with theselatter, a common mesoblastic origin. The simple arrangement ofthe cells in the earlier stages gradually gives place to the more com-plex disposition of the mature tissue. The development of endothelium forms part of the history ofthe changes taking place within the extensive mesodermic areas;from the specialized sheet, or mesothelium, bounding the primarybody-cavity of the young embryo, the endothelium of the pleural,pericardial, and peritoneal cavities directly descends, while the liningcells of the vascular and lymphatic channels trace their origin to thedifferentiation of certain of the mesodermic Endothelium from the septum cisternal of frog, stainedwith silver: a, one of the true stomata, lined withguard-cells; 6, intercellular cleft; «, nucleus. THE CONNECTIVE TISSUES. 35 CHAPTER III. THE CONNECTIVE TISSUES. The important group of connective substances—the most widely-distributed of all tissues—is the direct product of the great meso-blastic tract, axial as well as peripheral; the several members of thisextended family are formed by the differentiation and specializationof the intercellular substance, wrought through the more or lessdirect agency of the mesoblastic cells. The variation in the physicalcharacteristics of these substances is due to the condition of theintercellular constituents of the tissues. Taken during the periodof embryonal growth, they are represented by a semi-gelatinous,soft, plastic mass; a little later, the still soft, but already definitelyfo
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